In operation, piston engines, in particular piston internal combustion engines, are excited to vibrate as a function of the changing events in the cylinder chamber, such as the course of combustion, but also as a function of mechanical influences. The vibrations are also radiated as noise at the surfaces of the piston engine in the form of airborne noise, and/or are transmitted via the bearings of the piston engine into the substructure or into the body in vehicles as structure-borne sound.
Abatement of noise emissions of the above kind is sought, because of their negative effects on humans and the environment. DE-A-28 49 613 attempts to produce a noise shield by disposing an elastic acoustical insulation enclosure, which is attached to the engine block of a piston internal combustion engine. Furthermore, DE-A-28 01 431 suggests supporting the entire piston internal combustion engine in an outer tublike casing with the aid of support elements, which insulate structure-borne sound. A disadvantage of such an acoustical insulation measure is that it contains a large part of the machine and therefore hinders the installation of add-on parts and/or additional units, such as engine mounts, starter, generator, or gas supply lines and gas exhaust lines. In this connection, in many cases, it is impossible to prevent the breaching of acoustical insulation enclosures of the above kind in order to install add-on parts of the same kind and/or additional units, which measure reduces the effectiveness of the arrangement. Furthermore, acoustical insulation measures of the above kind reduce the heat tolerance of a piston internal combustion engine.
On account of the above mentioned disadvantages, there have been attempts to control noise propagation by seeking to prevent or at least to reduce the generation of noise. In addition to reducing sources of excitation, for example by optimizing the combustion process, it makes sense primarily to reduce the noise transmission and noise radiation at the surfaces of the piston engine. The above is achieved by configuring the piston engine to be as rigidly as possible, particularly making it resistant to bending, or torsionally rigid, especially in its thin-walled regions; the oscillatory faces are configured to be as small and/or thick-walled as possible with regard to airborne noise radiation. According to the above arrangement, however not only is there then an undesired increase in weight, particularly resulting from an increase in wall thickness, primarily in cast components, but increased casting defects such as bubbles or pores or the like also occur. DE-A-35 44 215 has already suggested improving the rigidity of the engine block as a whole with a system of reinforcement ribs on the side walls in the cylinder region. As a result, undesired casting defects can be prevented by configuring the ribs in this way, and high rigidity of the cylinder block can be achieved.
DE-A-40 17 139 suggests the concept of achieving the required rigidity of the engine block via the purposeful installation of bands and ribs. According to this proposal, the above is achieved in particular by binding the crankshaft bearings to the cylinder block and to the side walls of the crankcase via a multitude of reinforcing ribs, so that the rigidity of the engine block structure as a whole is increased. However, the above arrangement entails a corresponding increase in weight. However, from an economical standpoint, a weight increase is to be avoided.